High molecular weight, normally solid copolymers of ethylene and unsaturated carboxylic acids, such as acrylic acid and methacrylic acid, are well known. However, there is a need for new ethylene copolymers which have improved film optics, while maintaining other performance attributes.
There are two main reactor types to produce high pressure free radical copolymers of ethylene and unsaturated carboxylic acids, namely the autoclave reactor and the tubular reactor. Generally a tubular reactor is more advantaged for making narrow molecular weight distribution (MWD) polyethylene due the uniform residence time in tubular reaction zones; however in case of the manufacturing of ethylenic-carboxylic acid copolymer a tubular reactor is less suited as a first reaction zone due to the following.
1. The requirement to preheat the reactor feed to a minimum start temperature to avoid phase separation.
2. The risk of premature carboxylic acid comonomer polymerization and consequently gel formation in the heating step of the reactor feed.
3. The high inlet acid comonomer concentration in a tubular reactor enhances the risk of phase separation.
For these reasons an autoclave or a hybrid autoclave-tubular reactor system is preferred for the production of acid high pressure copolymers. However due to broadening of the MWD by the residence time distribution in an autoclave reactor, this reactor type is less suited for making narrow MWD copolymer products.
Good optical properties are achieved by making a narrow MWD polymer. The MWD can be narrowed by polymerizing at lower temperatures and or higher operating pressure. Typically the maximum operating pressure is limited by the design of the compression and/or reaction section. In practice the MWD of a copolymer is narrowed by lowering polymerization temperature conditions. However lowering the polymerization temperature increases the risk of inducing phase separation conditions.
There remains a need for the production of narrow MWD ethylenic-carboxylic acid copolymer products with low gel levels, by suppressing phase separation and or hydrogen bonding of the carboxylic acid groups in the reactor. These needs and others have been met by the following invention.